The invention relates to an upper end piece of a nuclear fuel assembly for a pressurized water reactor, which assembly comprises a lower end piece secured to a structural element belonging to the upper end piece by guide tubes for receiving a cluster of control elements.
The invention is particularly suitable for use in spectral shift reactors in which the energy spectrum of the neutrons in the core is modified as the fuel burns up. This modification is obtained by changing the proportion of moderator in the core, by means of clusters of elements whose role is to replace moderating water in some at least of the guide tubes of an assembly (French No. 82 18011).
The fuel assemblies of the above-defined type generally comprise means for damping the fall of the control element clusters upon shut down of the reactor. These means often operate by throttling outflow of water contained in the guide tubes when the control elements approach their lower position. The throttling action may be obtained by a restriction in the lower part of the guide tubes.
The damping means have the disadvantage of considerably complicating the manufacture of the guide tubes because of the accuracy required in adjusting the damping effect. To complete the damping effect of the head loss impressed to the outflow, it has been proposed to add resilient means which are compressed by the cluster at the end of movement (French Nos. 2 106 373 and 2 070 373). The resilient means are integrated in the device controlling the cluster and consequently must have a small volume and a correspondingly low damping effect. Consequently, they only have a truly significant effect when the cluster is of moderate weight and so comprises a small number of control elements.
The nuclear fuel assemblies for use in spectral shift reactors raise an additional problem, because each assembly must be adaptated for receiving a cluster of control elements controlling the reactor moderation rate in addition to the usual cluster of absorbing control elements or in place thereof.
It is an object of the invention to provide an improved upper end piece; it is a more specific object to provide an end piece for satisfactory damping during fall of the control sclusters or "scram", using resilient means whose design does not contradict space considerations.
To this end, there is provided an upper end piece for a nuclear fuel assembly which comprises an abutting member for receiving the cluster at the end of its fall, means for guiding said abutting member parallel to the axis of the guide tubes and resilient means within the end piece and disposed between the structural member and the abutting member for absorbing the energy of the cluster at the end of the fall thereof.
In the case of an end piece for a spectral shift reactor assembly, adapted for receiving a cluster of absorbent elements and/or a cluster of elements for modifying the moderation rate by driving out the water contained in guide tubes, the resilient means of the end piece may comprise two springs and two abutment elements each cooperating with one of the clusters, one of which is formed by a plate constituting said abutting member. The springs may be disposed so that only one of the two is compressed by the fall of a particular one of the clusters, whereas the two springs are compressed and provide a damping function upon fall of the other cluster. The two springs may be disposed coaxially to each other.
The end piece may be completed by additional resilient means located to receive an upper core plate and thus to complete the hold-down function of the coaxial springs, i.e. to oppose raising of the assembly by the upward flow of the water serving as moderator and coolant, during operation of the reactor.
In an advantageous embodiment of the invention, the end piece forms a block independent of the rest of the assembly which comprises, in addition to the lower end piece and guide tubes, a simple upper perforated table fixed permanently to the guide tubes. Thus it is possible to replace the end piece should it be damaged or, on the contrary, to use the end piece successively on several assemblies, the end piece being easily removed from a completely exhausted assembly or a defective assembly.
The invention will be better understood from reading the following description of particular embodiments given by way of non limitative examples.